Tilting pad thrust bearings contain a plurality of bearing pads which are each supported on a disk for tilting in a retainer ring on a hardened spherical pivot button which extends from the bottom of the disk. The pads are flooded with circulating lubricant. A rotating thrust collar or runner bears on the top surfaces of the pads and rides on an oil film on the top surface of the pads. The bearing pads are usually made of steel that is faced with a low friction material such as babbitt.
As the runner rotates, it shears the oil film that separates the runner from the bearing pad faces and generates heat at the surface of the pads. This results in a temperature differential across each pad and through each pad. The top surface is the hottest so it assumes a slightly convex shape. The convexity or downward bending of the leading and trailing edges of the pads distorts the oil film on the bearing surface and influences its load-bearing capacity. The load on the pad causes a further downward bending of the pad about its pivot point and this also affects load bearing capacity, but the bending due to temperature differences is normally several times greater than that due to pressure.
The term "leading edge" as used herein is the edge of a bearing pad that would be traversed first by any line on the runner moving in the direction of rotation or translation over the series of circularly arranged or linearly arranged bearing pads. The "trailing edge" then is the edge of the pad over which said point or line is second to pass in the direction of rotation or translation of the runner.
Because of hydrodynamic forces generated in the liquid lubricant, it has been found desirable to mount the pads for tilting on a point of contact between a spherical surface and a planar surface. The tilting action results in the maximum lubricant film thickness (h.sub.1) developing, of course, at the leading edge of the pads and a minimum film thickness (h.sub.2) developing at the trailing edge of the pads. In the design of tilting pad bearings, it is an objective, as in the case of the present invention, to reduce the film pressure, maximize film thickness and minimize the temperature of the lubricant film. The downward deflection or convexity developed by the top working surface of the bearing pads distorts the lubricating oil film between the bearing surface of the pad and the runner and causes, in general, a significant decrease in load-bearing capacity. In actual practice, the deflection for a babbitt-faced steel bearing pad is a few thousandths of an inch, generally varying with bearing size. This is a small amount but it affects the load bearing capacity of the bearing significantly.
Most tilting pad bearing assemblies adapted for use with a rotating runner have a plurality of sector or pie-shaped pads arranged in a circle. The theory of hydrodynamic film lubricated tilting pad thrust bearings is well known to those involved in designing and using large thrust bearings such as for hydraulic turbines and the like. The theory that has been generally accepted as valid indicates that maximum load capacity results when the pad pivot location is offset circumferentially in the direction of runner rotation to an optimum position approximately 0.6 of the length of the pad from its leading edge when the upper bearing surface of the pad is flat and the runner rotates in a single direction. In applications where the runner is rotationally reversible, the pivot point must necessarily be on a line that is centered or midway between the leading and trailing edges of the bearing pads. Theoretical analysis of the hydrodynamic properties of bearing pads that are supported centrally for bidirectional rotation and flat bearing surfaces indicates that an oil film would be developed that has no load bearing capacity at all. Theory and reality do not agree in this case. In actual practice the flat bearings develop some distortion due to heat and load which results in a load bearing capacity by the lubricant being developed. For unidirectional rotatable runners those skilled in the art have accepted establishing the pivot point for the tilting pads at 60% of the distance between their leading and trailing edges. In accordance with the present invention, however, much to the surprise of those who are involved in the design of heavy tilting pad bearings, it has been demonstrated recently by theoretical analysis and practical tests that when distortion is taken into account, the pivot point should be downstream from the leading edge by substantially more than 60% of the width of the bearing pad in accordance with the invention disclosed herein. This has been found to produce a thicker than heretofore obtained lubricant film near the trailing edge which is tantamount of saying that the bearing will have a higher load capacity.
A paper published by the United States Navy Department reports on tests made on two different types of thrust bearings one of which was a tilting pad bearing and the other was another type of thrust bearing. The performance characteristics were compared. The data show that performance of the tilting pad bearing improved as the point on which the pads pivot increased from 50% to 60% to 70% of the distance from the leading to the trailing edge of the pads. An optimized pivot point was not determined nor suggested. The traditional 60% pivot point was not positively challenged. The study was primarily for comparing two types of bearings. The publication is--Nathan T. Sides and Thomas L. Daugherty, "Performance Characteristics of Oil Lubricated Swing-Pad Thrust Bearings with Different Radii of Curvature"--Report No. DTNSRDC-80/122 (David Taylor Naval Ship Research and Development Center), published December, 1980. Government Accession No. AD-A093173.